Light emitting device

ABSTRACT

A light emitting device is provided which can prevent a change in gate voltage due to leakage or other causes and at the same time can prevent the aperture ratio from lowering. A capacitor storage is formed from a connection wiring line, an insulating film, and a capacitance wiring line. The connection wiring line is formed over a gate electrode and an active layer of a TFT of a pixel, and is connected to the active layer. The insulating film is formed on the connection wiring line. The capacitance wiring line is formed on the insulating film. This structure enables the capacitor storage to overlap the TFT, thereby increasing the capacity of the capacitor storage while keeping the aperture ratio from lowering. Accordingly, a change in gate voltage due to leakage or other causes can be avoided to prevent a change in luminance of an OLED and flickering of screen in analog driving.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an OLED (organic light emitting diode)panel in which an OLED formed over a substrate is sealed between thesubstrate and a sealing member. The invention also relates to an OLEDmodule obtained by mounting an IC to the OLED panel. In thisspecification, a light emitting device is used as the generic term forthe OLED panel and the OLED module. Also included in the presentinvention is electronic appliance using the light emitting device.

2. Description of the Related Art

Being self-luminous, OLEDs eliminate the need for a backlight that isnecessary in liquid crystal display devices (LCDs) and thus make it easyto manufacture thinner devices. Also, the self-luminous OLEDs are highin visibility and have no limit in terms of viewing angle. These are thereasons for attention that light emitting devices using the OLEDs arereceiving in recent years as display devices to replace CRTs and LCDs.

An OLED has a layer containing an organic compound that providesluminescence (electro luminescence) when an electric field is applied(the layer is hereinafter referred to as organic light emitting layer),in addition to an anode layer and a cathode layer. Luminescence obtainedfrom organic compounds is classified into light emission upon return tothe base state from singlet excitation (fluorescence) and light emissionupon return to the base state from triplet excitation (phosphorescence).A light emitting device according to the present invention can use oneor both types of light emission.

In this specification, all the layers that are provided between an anodeand a cathode is defined as an organic light emitting layer.Specifically, the organic light emitting layer includes a light emittinglayer, a hole injection layer, an electron injection layer, a holetransporting layer, an electron transporting layer, etc. A basicstructure of an OLED is a laminate of an anode, a light emitting layer,and a cathode layered in this order. The basic structure can be modifiedinto a laminate of an anode, a hole injection layer, a light emittinglayer, and a cathode layered in this order, or a laminate of an anode, ahole injection layer, a light emitting layer, an electron transportinglayer, and a cathode layered in this order.

One of methods of driving a light emitting device having an OLED isanalog driving in which an analog video signal is used.

In analog driving, an analog video signal is inputted to a gateelectrode of a TFT that controls a current flowing into the OLED(driving TFT). The amount of drain current of the driving TFT iscontrolled by the electric potential of the inputted analog videosignal. When the drain current flows into the OLED, the OLED emits lightat a luminance determined in accordance with the amount of the draincurrent. A gray scale is thus obtained.

A detailed description will be given with reference to FIG. 19 on howthe amount of current supplied to the OLED is controlled by the gatevoltage of the driving TFT in the above analog driving.

FIG. 19 is a graph showing a transistor characteristic of the drivingTFT. The characteristic is called I_(DS)-V_(GS) characteristic (orI_(DS)-V_(GS) curve). I_(DS) represents the drain current and V_(GS)represents the voltage between the gate electrode and the source region(gate voltage). V_(TH) represents the threshold voltage and V∞ meansthat V_(GS) is infinite. From the graph, one can tell how much currentflows when the gate voltage takes an arbitrary value.

An exclusive relation is formed between gate voltage and drain currentin accordance with the I_(DS)-V_(GS) characteristic shown in FIG. 19. Inother words, the drain current is determined in accordance with theelectric potential of an analog video signal inputted to the gateelectrode of the driving TFT. The drain current flows into the OLED,which emits light at a luminance determined in accordance with theamount of the drain current.

When the voltage between the source region and the drain region is givenas V_(DS), the transistor characteristic of the driving TFT which isshown in FIG. 19 is divided into two ranges by values of V_(GS) andV_(DS). A range where |V_(GS)-V_(TH)|<|V_(DS)| is satisfied is asaturation range, whereas a range where |V_(GS)-V_(TH)|>|V_(DS)| issatisfied is a linear range.

The following equation 1 is satisfied in the saturation range.I _(DS)=β(V _(GS) −V _(TH))²/2   Equation 1wherein β=μC_(O)W/L, μ represents the mobility, C_(O) represents thegate capacitance per unit area, and W/L represents the ratio of achannel width W to a channel length L of a channel formation region.

As Equation 1 shows, the current value in the saturation range is hardlychanged by V_(DS) and is determined solely by V_(GS). Therefore, it isrelatively easy to control the gray scale by the electric potential ofan analog signal. Accordingly, the driving TFT is operated mainly in thesaturation range in analog driving in general.

In the saturation range, however, a change in gate voltage causes anexponential change in drain current as is apparent in FIG. 19. For thatreason, the drain current in analog driving could be changed greatly bythe slightest change in gate voltage due to leakage or other causesduring a period started with input of an analog video signal and endingwith input of the next analog video signal. A great change in draincurrent is accompanied by a great change in luminance of the OLED. Thiscan therefore lead to a problem of flickering of screen, depending onthe frame frequency.

In order to avoid the problem above, it is important to hold the gatevoltage securely. Increasing the capacity of the storage capacitor canbe one of measures for holding the gate voltage securely. However, whenthe storage capacitor is increased, the aperture ratio is lowered toreduce the area of a pixel where light emission is actually obtained(area of effective light emission). The term area of effective lightemission refers to the area of a region in which light emitted from anOLED is not blocked by objects that do not transmit light, such as a TFTand wiring line formed on the substrate.

In recent years in particular, demands for images of higher definitionare increasing and how to solve the problem of lowered aperture ratiowhich accompanies enhancement of pixel definition is becoming everimportant. Accordingly, increasing the area that a storage capacitoroccupies in a pixel is not desirable.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems above, andan object of the present invention is therefore to provide a lightemitting device which can prevent a change in gate voltage due toleakage or other causes and at the same time can prevent the apertureratio from lowering.

In order to attain the above object, the present invention uses aconnection wiring line, an insulating film, and a capacitance wiringline to form a storage capacitor. The connection wiring line is formedover a gate electrode and an active layer of a TFT of a pixel, and isconnected to the active layer. The insulating film is formed on theconnection wiring line. The capacitance wiring line is formed on theinsulating film. Alternatively, the capacitance wiring line may beformed on the same interlayer insulating film on which a pixel electrodeis formed. In this case, the capacitance wiring line and the pixelelectrode may be formed from the same conductive film. A power supplyline may double as the capacitance wiring line.

This structure enables the storage capacitor to overlap the TFT, therebyincreasing the capacity of the storage capacitor while keeping theaperture ratio from lowering. Accordingly, a change in gate voltage dueto leakage or other causes can be controlled to prevent a change inluminance of an OLED and flickering of screen in analog driving.

Keeping the aperture ratio from lowering also leads to preventing thearea of effective light emission of a pixel from being reduced. As thearea of effective light emission is larger, the luminance of the screenis higher. Therefore, the structure of the present invention iseffective in reducing power consumption.

The structure of the present invention may also be used in digitaldriving.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram of a pixel in a light emitting device of thepresent invention;

FIG. 2 is a sectional view of a pixel in a light emitting device of thepresent invention;

FIGS. 3A to 3D are diagrams showing a process of manufacturing a lightemitting device of the present invention;

FIG. 4 is a top view of a light emitting device of the presentinvention;

FIGS. 5A to 5D are diagrams showing a process of manufacturing a lightemitting device of the present invention;

FIG. 6 is a top view of a light emitting device of the presentinvention;

FIGS. 7A and 7B are diagrams showing a process of manufacturing a lightemitting device of the present invention;

FIG. 8 is a top view of a light emitting device of the presentinvention;

FIG. 9 is a top view of a light emitting device of the presentinvention;

FIG. 10 is a sectional view of a pixel in a light emitting device of thepresent invention;

FIG. 11 is a sectional view of a pixel in a light emitting device of thepresent invention;

FIG. 12 is a sectional view of a pixel in a light emitting device of thepresent invention;

FIG. 13 is a sectional view of a pixel in a light emitting device of thepresent invention;

FIG. 14 is a circuit diagram of a pixel portion in a light emittingdevice of the present invention;

FIG. 15 is a timing chart in analog driving;

FIG. 16 is a timing chart in digital driving;

FIGS. 17A to 17C are diagrams of a light emitting device of the presentinvention, with FIG. 17A showing a top view thereof and FIGS. 17B and17C showing sectional views thereof;

FIGS. 18A to 18H are diagrams showing electronic appliance using a lightemitting device of the present invention;

FIG. 19 is a graph showing a transistor characteristic of a driving TFT;

FIGS. 20A and 20B are sectional views of a pixel in a light emittingdevice of the present invention; and

FIG. 21 is a sectional view of a pixel in a light emitting device of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode

The structure of the present invention will be described below.

A light emitting device of the present invention has a pixel portion inwhich a plurality of pixels form a matrix. A description is given withreference to FIG. 1 on a connection structure in a TFT of a pixel of thepresent invention.

A region that has one of source lines (S), one of gate lines (G), andone of power supply lines (V) corresponds to a pixel 100. Each pixel hasa switching TFT 101, a driving TFT 102, an OLED 103, and a storagecapacitor 104.

A gate electrode of the switching TFT 101 is connected to the gate line(G). The switching TFT 101 has a source region and a drain region one ofwhich is connected to the source line (S) and the other of which isconnected to a gate electrode of the driving TFT 102.

The driving TFT has a source region and a drain region one of which isconnected to the power supply line (V) and the other of which isconnected to a pixel electrode of the OLED 103. If an anode of the OLED103 is used as the pixel electrode, a cathode thereof is called anopposite electrode. If the cathode of the OLED 103 is used as the pixelelectrode, on the other hand, the anode is called the oppositeelectrode.

The switching TFT 101 may be either a p-channel TFT or n-channel TFT.The driving TFT 102 may be either a p-channel TFT or n-channel TFT.However, it is desirable to use a p-channel TFT for the driving TFT whenthe anode serves as the pixel electrode. On the other hand, when thecathode serves as the pixel electrode, the driving TFT is desirably ann-channel TFT.

Of two electrodes that the storage capacitor has, one is electricallyconnected to the gate electrode of the driving TFT 102 and the other iselectrically connected to the power supply line (V).

Next, a specific structure of the storage capacitor in the lightemitting device of the present invention will be described withreference to FIG. 2. Denoted by 101 and 102 are a switching TFT and adriving TFT, respectively. The TFTs are formed on an insulating surface.

The switching TFT 101 has an active layer 130, which has impurityregions 110 and 111. The impurity regions are to function as a sourceregion or a drain region. A gate electrode 114 is formed above theactive layer 130 with a gate insulating film 116 interposedtherebetween.

The driving TFT 102 has an active layer 131, which has impurity regions112 and 113. The impurity regions are to function as a source region ora drain region. A gate electrode 115 is formed above the active layer131 with the gate insulating film 116 interposed therebetween.

A first interlayer insulating film 133 and a second interlayerinsulating film 117 are formed to cover the active layers 130 and 131and the gate electrodes 114 and 115 of the switching TFT 101 and thedriving TFT 102, and the gate insulating film 116. Although twointerlayer insulating films, namely, the first interlayer insulatingfilm 133 and the second interlayer insulating film 117, are formed inFIG. 2, the number of interlayer insulating films may be one. Formed onthe second interlayer insulating film 117 are a source line (S),connection wiring lines 118 and 119, and a power supply line (V).

The source line (S) is connected to the impurity region 110 through acontact hole that is formed in the first interlayer insulating film 133,the second interlayer insulating film 117, and a gate insulating film116. The connection wiring line 118 is connected to the impurity region111 through a contact hole that is formed in the first interlayerinsulating film 133, the second interlayer insulating film 117, and agate insulating film 116.

The connection wiring line 119 is connected to the impurity region 112through a contact hole that is formed in the first interlayer insulatingfilm 133 and the second interlayer insulating film 117. The power supplyline (V) is connected to the impurity region 113 through a contact holethat is formed in the first interlayer insulating film 133 and thesecond interlayer insulating film 117.

The connection wiring line 118 overlaps the active layer 130 with thesecond interlayer insulating film 117, the first interlayer insulatingfilm 133, and a gate insulating film 116 interposed therebetween.

A third interlayer insulating film 120 is formed on the secondinterlayer insulating film 117 so as to cover the source line (S), theconnection wiring lines 118 and 119, and the power supply line (V). Onthe third interlayer insulating film 120, a capacitance wiring line 121and a pixel electrode 122 are formed.

The pixel electrode 122 is connected to the connection wiring line 119through a contact hole that is formed in the third interlayer insulatingfilm 120.

In the present invention, the storage capacitor 104 is formed in an areawhere the third interlayer insulating film 120 is sandwiched between theconnection wiring line 118 and the capacitance wiring line 121. Thecapacitance wiring 121 can be formed from the same conductive film asthe pixel electrode 122 and, therefore, the storage capacitor can beformed without increasing the number of steps in manufacture process.The storage capacitor 104 is formed to overlap the active layer 130 ofthe switching TFT 101, which makes it possible to obtain a storagecapacitor without reducing the aperture ratio.

A fourth interlayer insulating film 125 is formed on the thirdinterlayer insulating film 120 so as to cover the capacitance wiringline 121 and the pixel electrode 122. The fourth interlayer insulatingfilm 125 is partially etched to expose the pixel electrode 122.

An opposite electrode 124 is layered on an organic light emitting layer123 so as to cover the pixel electrode 122 and the fourth interlayerinsulating film 125. An area where the pixel electrode 122, the organiclight emitting layer 123, and the opposite electrode 124 overlapcorresponds to the OLED 103.

In the present invention, structures of the TFTs are not limited tothose shown in FIG. 2. Also, the present invention may have, in additionto the storage capacitor 104 that is formed from the connection wiringline 118 and the capacitance wiring line 121, a storage capacitor ofdifferent structure.

In the pixel structure according to the present invention, theconnection wiring line 118 is formed so as to overlap the active layerof the switching TFT 101. Therefore, OFF current is prevented fromflowing in the switching TFT 101 when light emitted from the OLED, orlight made incident from the outside of the light emitting device, isincident on the active layer 130.

Shown in FIG. 2 is the case where the switching TFT 101 is an n-channelTFT and the driving TFT 102 is a p-channel TFT, but the presentinvention is not limited thereto. The switching TFT 101 may be either ap-channel TFT or n-channel TFT, and the same applies to the driving TFT102. However, it is desirable to use a p-channel TFT for the driving TFTin FIG. 2 since the pixel electrode 122 here serves as the anode.

This embodiment mode shows a case in which a pixel has two TFTs.However, the present invention is not limited thereto. A storagecapacitor structured in accordance with the present invention can beformed irrespective of how many TFTs one pixel has. The storagecapacitor of the present invention can be obtained as long as it isformed from: a wiring line (connection wiring line) that is formed overa gate electrode and an active layer of a TFT of a pixel and isconnected to the active layer; an insulating film formed on theconnection wiring line; and a wiring line (capacitance formed on theinsulating film.

With the above structure, the present invention can make the storagecapacitor overlap the TFT and therefore can increase the capacity of thestorage capacitor while keeping the aperture ratio from lowering.Accordingly, a change in gate voltage due to leakage or other causes canbe avoided to thereby prevent a change in luminance of the OLED andflickering of screen in analog driving.

Keeping the aperture ratio from lowering also leads to preventing thearea of effective light emission of a pixel from being reduced. As thearea of effective light emission is larger, the luminance of the screenis higher. Therefore, the structure of the present invention iseffective in reducing power consumption.

Embodiments of the present invention will be described below.

Embodiment 1

A description is given with reference to FIGS. 3A to 8 on an example ofa method of manufacturing a light emitting device according to thepresent invention. The description is given step by step about detailsof a method of manufacturing TFTs in the pixel shown in FIG. 1.

First, a glass substrate 200 is prepared. Barium borosilicate glass,typical example of which is Corning #7059 glass or #1737 glass (productof Corning Incorporated), or alumino borosilicate glass is usable as thesubstrate 200. The substrate 200 can be any light-transmissivesubstrate, and a quartz substrate may also be used. A plastic substratemay be employed if it has heat resistance against the processtemperature of this embodiment.

Next, as shown in FIG. 3A, a base film 201 is formed on the substrate200 from an insulating film such as a silicon oxide film, a siliconnitride film, or a silicon oxynitride film. In this embodiment, the basefilm 201 has a two-layer structure. However, a single layer or more thantwo layers of the insulating films listed above may be used as the basefilm. The first layer of the base film 201 is a silicon oxynitride film201 a formed to have a thickness of 10 to 200 nm (preferably 50 to 100nm) by plasma CVD using as reaction gas SiH₄, NH₄, and N₂O. The siliconoxynitride film 201 a (composition ratio: Si=32%, O=27%, N=24%, H=17%)formed in this embodiment has a thickness of 50 nm. The second layer ofthe base film 201 is a silicon oxynitride film 201 b formed to have athickness of 50 to 200 nm (preferably 100 to 150 nm) by plasma CVD usingas reaction gas SiH₄ and N₂O. The silicon oxynitride film 201 b(composition ratio: Si=32%, O=59%, N=7%, H=2%) formed in this embodimenthas a thickness of 100 nm.

On the base film 201, semiconductor layers 202 to 204 are formed. Thesemiconductor layers 202 to 204 are formed by patterning into a desiredshape a crystalline semiconductor film that is obtained by forming asemiconductor film with an amorphous structure through a known method(sputtering, LPCVD, plasma CVD, or the like) and then subjecting thefilm to known crystallization treatment (e.g., laser crystallization,thermal crystallization, or thermal crystallization using nickel orother catalysts). The semiconductor layers 202 to 204 are each 25 to 80nm in thickness (preferably 30 to 60 nm). The material of thecrystalline semiconductor film is not limited but silicon or a silicongermanium (Si_(X)Ge_(1-X) (X=0.0001 to 0.02)) alloy is preferable. Inthis embodiment, an amorphous silicon film with a thickness of 55 nm isformed by plasma CVD and then a solution containing nickel is held tothe top face of the amorphous silicon film. The amorphous silicon filmis subjected to dehydrogenation (at 500° C., for an hour), then tothermal crystallization (at 550° C., for four hours), and then to laserannealing treatment for improvement of crystallinity to obtain acrystalline silicon film. Patterning treatment using photolithography isconducted on this crystalline silicon film to form the semiconductorlayers 202 to 204.

The semiconductor layers 202 to 204 may be doped with a minute amount ofimpurity element (boron or phosphorus) in order to control the thresholdof the TFTs.

If laser crystallization is used to form the crystalline semiconductorfilm, a pulse oscillating or continuous wave excimer laser, YAG laser,or YVO₄ laser can be employed. Laser light emitted from these laseroscillators is preferably collected into a linear beam by an opticalsystem before irradiating the semiconductor film. Though conditions ofcrystallization can be set suitably by an operator, there are somepreferred conditions. When an excimer laser is used, preferableconditions include setting the pulse oscillation frequency to 300 Hz,and the laser energy density to 100 to 400 mJ/cm² (typically, 200 to 300mJ/cm²). When a YAG laser is used, preferable conditions include usingthe second harmonic thereof, and setting the pulse oscillation frequencyto 30 to 300 kHz and the laser energy density to 300 to 600 mJ/cm²(typically, 350 to 500 mJ/cm²). The laser light is collected into alinear beam having a width of 100 to 1000 μm, 400 μm, for example, toirradiate the entire surface of the substrate with the beam. In theirradiation, the overlap ratio of the linear laser light is set to 50 to98%.

Next, a gate insulating film 205 is formed to cover the semiconductorlayers 202 to 204. The gate insulating film 205 is formed from aninsulating film containing silicon by plasma CVD or sputtering to have athickness of 40 to 150 nm. This embodiment uses a silicon oxynitridefilm (composition ratio: Si=32%. O=59%, N=7%, H=2%) formed by plasma CVDto have a thickness of 110 nm. The gate insulating film is not limitedto the silicon oxynitride film, of course, but may be a single layer ora laminate of other insulating films containing silicon.

When a silicon oxide film is used for the gate insulating film, the filmis formed by plasma CVD in which TEOS (tetraethyl orthosilicate) and O₂are mixed, the reaction pressure is set to 40 Pa, the substratetemperature is set to 300 to 400° C., and the high frequency (13.56 MHz)power density is set to 0.5 to 0.8 W/cm² for electric discharge. Thesilicon oxide film thus formed can provide excellent characteristics asthe gate insulating film when the film receives subsequent thermalannealing at 400 to 500° C. The device that has finished the steps abovepresents the sectional view shown in FIG. 3A.

Resist masks 206 are formed next and the semiconductor layers are dopedwith an n type impurity element (phosphorus, in this embodiment) to formimpurity regions 207 to 209 that contain high concentration ofphosphorus. These regions each contain phosphorus in a concentration of1×10²⁰ to 5×10²¹ atoms/cm³, typically, 2×10²⁰ to 1×10²² atoms/cm³ (FIG.3B).

A heat-resistant conductive layer for forming a gate electrode is formedon the gate insulating film 205 (FIG. 3C). A heat-resistant conductivelayer 210 may be a single layer or a laminate of two, three, or morelayers if necessary. In this embodiment, a laminate of a conductive film(A) 210 a and a conductive film (B) 210 b makes the heat-resistantconductive layer. The heat-resistant conductive layer may be a filmcontaining an element selected from the group consisting of tantalum(Ta), titanium (Ti), molybdenum (Mo), tungsten (W), chromium (Cr), andsilicon (Si). Alternatively, the heat-resistant conductive layer may bea conductive film mainly containing one of the elements listed above(typically a tantalum nitride film, a tungsten nitride film, a titaniumnitride film, or the like), or an alloy film containing a combination ofthe elements listed above (typically a Mo—W alloy film, a Mo—Ta alloyfilm, a tungsten silicide film, or the like). This embodiment uses a TaNfilm for the conductive film (A) 210 a and a W film for the conductivefilm (B) 210 b. These films that constitute the heat-resistantconductive layer are formed by sputtering or CVD. In order to reduce theresistance of the films, the concentration of impurities contained inthe films should be lowered and the oxygen concentration in particularis preferably reduced to 30 ppm or less. The W film may be formed bysputtering with W as the target, or by thermal CVD using tungstenhexafluoride (WF₆). In either case, the W film has to have a lowresistivity in order to use the W film as a gate electrode. A desirableresistivity of the W film is 20 μΩcm or lower. The resistivity of the Wfilm can be reduced by increasing the crystal grain size but, if thereare too many impurity elements such as oxygen in the W film,crystallization is inhibited to raise the resistivity. Accordingly, whenthe W film is formed by sputtering, a W target with a purity of 99.99%or 99.9999% is used and a great care is taken not to allow impurities inthe air to mix in the W film that is being formed. As a result, the Wfilm can have a resistivity of 9 to 20 μΩcm.

Sputtering can also be used to form a Ta film for the heat-resistantconductive layer 210. The Ta film is formed by using Ar as sputteringgas. If an appropriate amount of Xe or Kr is added to the sputteringgas, the internal stress of the obtained Ta film is eased to prevent theTa film from peeling off. The resistivity of a Ta film in α phase isabout 20 μΩcm and is usable as a gate electrode. On the other hand, theresistivity of a Ta film in P phase is about 180 μΩcm and is notsuitable for a gate electrode. A Ta film in β phase can readily beobtained by forming, as a base of a Ta film, a TaN film that has acrystal structure approximate to that of the α phase. Though not shownin the drawings, it is effective to form a silicon film doped withphosphorus (P) to have a thickness of 2 to 20 nm under theheat-resistant conductive layer 210. This improves adherence to theconductive film to be formed thereon and prevents oxidization. At thesame time, the silicon film prevents a minute amount of alkaline metalelement contained in the heat-resistant conductive layer 210 fromdiffusing into the first shape gate insulating film 205. Whatevermaterial is used, a preferable resistivity range for the heat-resistantconductive layer 210 is 10 to 50 μΩcm.

The conductive film (A) 210 a and the conductive film (B) 210 b are thenpatterned into desired shapes to form gate electrodes 211 and 212 and acapacitance electrode 213 (FIG. 3D). Though not clear in FIG. 3D, thecapacitance electrode 213 is connected to the gate electrode 212.

A top view of the pixel that has finished the step of FIG. 3D is shownin FIG. 4. FIG. 3D corresponds to the sectional view of the pixel takenalong the line A-A′ in FIG. 4. Note that the gate insulating film 205 isomitted here for clearer view. Denoted by 250 is gate line that isconnected to the gate electrode 211.

Using the gate electrode 211 as a mask, the semiconductor layers 202 and203 hat are to serve as active layers of TFTs are doped with an impurityelement for imparting the n type conductivity (hereinafter referred toas n type impurity element). Elements usable as the n type impurityelement are ones belonging to Group 15 in the periodic table, typically,phosphorus or arsenic. Formed through this doping step are firstimpurity regions 215 to 217, 220, and 221, a second impurity region 218,and channel formation regions 219 and 222. One of the first impurityregions 215 and 217 functions as a source region whereas the otherfunctions as a drain region. The second impurity region 218 is a lowconcentration impurity region that functions as an LDD region, andcontains the n type impurity element in a concentration of 1×10¹⁶ to5×10²⁰ atoms/cm³, typically, 1×10¹⁷ to 5×10¹⁸ atoms/cm³ (FIG. 5A).

The regions for forming n-channel TFTs are covered with masks 223 todope the semiconductor layer 203 that is to serve as an active layer ofa p-channel TFT with boron as a p type impurity element. Theconcentration of boron is set to 3×10²⁰ to 3×10²¹ atoms/cm³, typically,5×10²¹ atoms/cm³ (FIG. 5B). Through this step, third impurity regions224 and 225 are formed in the semiconductor layer 203.

Next, a first interlayer insulating film 226 is formed on the gateelectrodes 211 and 212, the capacitance electrode 213, and the gateinsulating film 205. The first interlayer insulating film 226 may be asilicon oxide film, a silicon oxynitride film, or a silicon nitridefilm, or may be a laminate having these insulating films in combination.In any case, the first interlayer insulating film 226 is formed from aninorganic insulating material. The thickness of the first interlayerinsulating film 226 is set to 100 to 200 nm. If a silicon oxide film isused for the first interlayer insulating film 226, the film is formed byplasma CVD in which TEOS and O₂ are mixed, the reaction pressure is setto 40 Pa, the substrate temperature is set to 300 to 400° C., and thehigh frequency (13.56 MHz) power density is set to 0.5 to 0.8 W/cm² forelectric discharge. If a silicon oxynitride film is used for the firstinterlayer insulating film 226, the film may be formed by plasma CVDfrom SiH₄, N₂O, and NH₃, or from SiH₄ and N₂O. The film formationconditions in this case include setting the reaction pressure to 20 to200 Pa, the substrate temperature to 300 to 400° C., and the highfrequency (60 MHz) power density to 0.1 to 1.0 W/cm². The firstinterlayer insulating film 226 may be a silicon oxynitride hydrate filmformed from SiH₄, N₂O, and H₂. A silicon nitride film as the firstinterlayer insulating film can be formed similarly by plasma CVD fromSiH₄ and NH₃.

Then an activation step is conducted to activate the impurity elementsthat are used to dope the semiconductor layers in differentconcentrations and impart them the n type or p type conductivity (FIG.5C). The conductive films used as the gate electrodes in this embodimentare easily oxidized and the resistivity thereof is raised as a result ofoxidization. Accordingly, heat treatment for activation in thisembodiment is conducted preferably under reduced pressure atmosphere byevacuation using a rotary pump or mechanical booster pump to reduce theoxygen concentration in the atmosphere.

Next, dangling bonds in the active layers are terminated throughhydrogenation using thermally excited hydrogen. The hydrogenation isachieved by heat treatment in a hydrogen atmosphere at 410° C. for anhour. Other hydrogenation means include plasma hydrogenation that useshydrogen excited by plasma.

Then, a second interlayer insulating film 227 is formed to have athickness of 500 to 1000 nm (800 nm, in this embodiment). The secondinterlayer insulating film 227 may be an organic insulating film such asan acrylic film, a polyimide film, a polyamide film, or a BCB(benzocyclobutene) film, or an inorganic insulating film such as asilicon oxynitride film or a silicon nitride oxide film.

Thereafter, a resist mask of a given pattern is formed to form contactholes reaching the first impurity regions 215 and 217, the thirdimpurity regions 224 and 225, and the impurity region 209. The contacthole reaching the impurity region 209 is omitted in FIG. 5D. The contactholes are formed by dry etching. In this case, a mixture of CF₄, O₂, andHe is used as etching gas to etch the second interlayer insulating film227 first. The etching gas is then changed to a mixture of CF₄ and O₂ toetch the first interlayer insulating film 226. The etching gas isfurther changed to CHF₃ in order to enhance the selective ratio with thesemiconductor layers, and the gate insulating film 205 is etched. Thus,the contact holes are formed.

A metal conductive film is formed by sputtering or vacuum evaporationand patterned using a mask. The film is then etched to form a sourceline 228, connection wiring lines 229 and 230, and a power supply line231. The source line 228 is connected to the first impurity region 215.The connection wiring line 229 is connected to the first impurity region217. The connection wiring line 230 is connected to the third impurityregion 224. The power supply line 231 is connected to the third impurityregion 225. Though not shown in FIG. 5D, the connection wiring line 229is connected to the gate electrode 212 and the power supply line 231 isconnected to the impurity region 209.

The wiring lines in this embodiment are formed from a laminate of a Tifilm with a thickness of 50 nm and an alloy film (Al—Ti alloy film) witha thickness of 500 nm, though not shown in FIG. 5D.

A top view of the pixel that has finished the step of FIG. 5D is shownin FIG. 6. FIG. 5D corresponds to the sectional view of the pixel takenalong the line A-A′ in FIG. 6. Note that the gate insulating film 205and the first and second interlayer insulating film 226 and 227 areomitted here for clearer view. Denoted by 250 is a gate line.

The connection between the connection wiring line 229 and the gateelectrode 212 is illustrated in FIG. 20A. FIG. 20A corresponds to thesectional view of the pixel taken along the line B-B′ in FIG. 6. Theconnection wiring line 229 is connected to the gate electrode 212through the contact hole that is formed in the second interlayerinsulating film 227 and the first interlayer insulating film 226.

The connection between the power supply line 231 and the impurity region209 is illustrated in FIG. 20B. FIG. 20B corresponds to the sectionalview of the pixel taken along the line C-C′ in FIG. 6. The power supplyline 231 is connected to the impurity region 209 through the contacthole that is formed in the second interlayer insulating film 227 and thefirst interlayer insulating film 226.

A third interlayer insulating film 233 is formed next. For the need ofplanarization, the third interlayer insulating film 233 is formed froman organic insulating film such as a polyimide film or an acrylic filmto have a thickness of 1.5 μm. A contact hole reaching the connectionwiring line 230 is formed in the third interlayer insulating film 233. Atransparent conductive film with a thickness of 80 to 120 nm is formedon the third interlayer insulating film 233 and then patterned. Thusformed are a pixel electrode 234 and a capacitance wiring line 235 (FIG.7A). The transparent conductive film used in this embodiment is anindium tin oxide (ITO) film or a film obtained by mixing 2 to 20% ofzinc oxide (ZnO) with indium oxide.

The capacitance wiring line 235 overlaps the connection wiring line 229with the third interlayer insulating film 233 interposed therebetween.In the present invention, a storage capacitor 236 is formed from thecapacitance wiring line 235, the third interlayer insulating film 233,and the connection wiring line 229.

A top view of the pixel that has finished the step of FIG. 7A is shownin FIG. 8. FIG. 7A corresponds to the sectional view of the pixel takenalong the line A-A′ in FIG. 8. Note that the third interlayer insulatingfilm 233 is omitted here for clearer view.

Though not shown in FIG. 7A, the capacitance wiring 235 that constitutesthe storage capacitor 236 is connected to the capacitance wiring line235 of the adjacent pixel. FIG. 9 shows how a plurality of pixels, eachstructured as illustrated in FIG. 8, are arranged.

The source line and the power supply line are denoted by 228 and 231,respectively. As shown in FIG. 9, the capacitance wiring line 235 in onepixel is connected to the capacitance wiring line 235 of its adjacentpixel. Alternatively, adjacent pixels share one capacitance wiring line235. Every capacitance wiring line 235 receives a given electricpotential. Denoted by 250 is the gate line that is connected to the gateelectrode 211.

Next, a fourth interlayer insulating film 237 having an opening at theposition that coincides with the pixel electrode 234 is formed as shownin FIG. 7B. The fourth interlayer insulating film 237 is capable ofinsulating, and functions as a bank to separate organic light emittinglayers of adjacent pixels from one another. In this embodiment, a resistis used to form the fourth interlayer insulating film 237.

An organic light emitting layer 238 is then formed by evaporation. Acathode (MgAg electrode) 239 and a protective electrode 240 are alsoformed by evaporation. Desirably, heat treatment is conducted on thepixel electrode 234 prior to formation of the organic light emittinglayer 238 and the cathode 239, so that moisture contained is removedcompletely. Although a MgAg electrode is used for the cathode of theOLED in this embodiment, other known materials may be used instead.

A known material can be used for the organic light emitting layer 238.The organic light emitting layer in this embodiment has a two-layerstructure consisting of a hole transporting layer and a light emittinglayer. The organic light emitting layer may additionally have one orsome of a hole injection layer, an electron injection layer, and anelectron transporting layer. Various combinations have been reported andthe organic light emitting layer of this embodiment can take any ofthose.

The hole transporting layer of this embodiment is formed by evaporationfrom polyphenylene vinylene. The light emitting layer of this embodimentis formed by evaporation from polyvinyl carbazole with 30 to 40% of PBD,that is a 1,3,4-oxadiazole derivative, being molecule-dispersed. Thelight emitting layer is doped with about 1% of Coumarin 6 as greenluminescent center.

The protective electrode 240 alone can protect the organic lightemitting layer 238 from moisture and oxygen, but it is more desirable touse a protective film 241. This embodiment uses a silicon nitride filmwith a thickness of 300 nm as the protective film 241. The protectiveelectrode 240 and the protective film may be formed in successionwithout exposing the device to the air.

The protective electrode 240 also prevents degradation of the cathode239. A typical material of the protective electrode is a metal filmmainly containing aluminum. Other materials may of course be used. Sincethe organic light emitting layer 238 and the cathode 239 are extremelyweak against moisture, the organic light emitting layer, the cathode,and the protective electrode 240 are desirably formed in successionwithout exposing them to the air. The organic light emitting layer andthe cathode are thus protected from the outside air.

The organic light emitting layer 238 is 10 to 400 nm in thickness(typically 60 to 150 nm), and the cathode 239 is 80 to 200 nm inthickness (typically 100 to 150 nm).

Thus completed is a light emitting device structured as shown in FIG.7B. An area 242 where the pixel electrode 234, the organic lightemitting layer 238, and the cathode 239 overlap corresponds to the OLED.

In this embodiment, a storage capacitor 243 is formed from the impurityregion 209, the gate insulating film 205, and the capacitance electrode213. A storage capacitor 244 is formed from the capacitance electrode213, the second interlayer insulating film 227, and the power supplyline 231. The impurity region 209 and the capacitance electrode 213overlap the power supply line 231 and, therefore, the storage capacitors243 and 244 can be formed without lowering the aperture ratio.

Denoted by 245 and 246 are a switching TFT and a driving TFT,respectively.

In practice, the device that has reached the stage of FIG. 7B ispreferably packaged (sealed) to further avoid exposure to the outsideair. For packaging, a protective film which is highly airtight and whichallows little gas to leak (such as a laminate film or a UV-curable resinfilm) or a light-transmissive sealing member can be used. Thereliability of the OLED is improved if the interior of the sealingmember has an inert atmosphere or a hygroscopic material (barium oxide,for example) is placed inside.

The light emitting device of the present invention can be made by othermethods than the manufacture method described in this embodiment. Aknown method can be used to manufacture the light emitting device of thepresent invention.

Embodiment 2

This embodiment describes a storage capacitor of the present inventionwhich has a structure different from the one in FIG. 7A.

FIG. 10 is a sectional view of a pixel of this embodiment. Referencesymbol 301 denotes a switching TFT, and 302, a driving TFT. Theswitching TFT and the driving TFT here are an n-channel TFT and ap-channel TFT, respectively, but this embodiment is not limited thereto.The switching TFT may be either a p-channel TFT or n-channel TFT, andthe same applies to the driving TFT.

After a second interlayer insulating film 303 is formed, contact holesare formed in the second interlayer insulating film 303, a gateinsulating film 307, and a first interlayer insulating film 306. Formednext is a conductive layer for forming connection wiring lines 305 and320, a source line 304, and a power supply line 321. The conductivelayer in this embodiment has a laminate structure in which a 300 to 500nm thick conductive film mainly containing aluminum (Al) is laid on a 50to 100 nm thick conductive film mainly containing titanium (Ti). Aconductive film for forming connection wiring lines is a laminate of acombination of a film mainly containing tantalum (Ta), a conductive filmmainly containing aluminum (Al), or a film mainly containing titanium(Ti).

On the surface of the conductive layer, an insulating film 310 is formedby anodization or plasma oxidization (anodization, in this embodiment)to have a thickness of 20 to 100 nm (preferably 30 to 50 nm). Theinsulating film serves as dielectric. In this embodiment, the connectionwiring line 305 is a laminate of a film mainly containing aluminum and afilm mainly containing titanium, and the film mainly containing aluminumis anodized to form an aluminum oxide film (alumina film) that is ananodized film. The anodized film in this embodiment corresponds to theinsulating film 310, and is used as the dielectric of the storagecapacitor. An insulating oxide film obtained by anodization of atantalum (Ta) film or a titanium (Ti) film also has high dielectricconstant and therefore is suitable as the dielectric of the storagecapacitor.

For the anodization treatment, a tartaric acid ethylene glycol solutionwith a low enough alkaline ion concentration is prepared first. Thesolution is obtained by mixing a 15% tartaric acid ammonium aqueoussolution and ethvlene glycol at a ratio of 2:8. Ammonia water is addedto the mixture to adjust pH thereof to 7±0.5. A platinum electrode thatserves as a cathode is set in the thus prepared solution, and thesubstrate on which the conductive layer is formed is immersed in thesolution. The conductive layer works as an anode and a direct current ofa given amount (several mA to several tens mA) is caused to flow. Inthis embodiment, a 200 mA current flows in one substrate.

Though the voltage between the cathode and the anode in the solutionchanges with time as the anodized substance grows, the voltage is raisedat a given rate until it reaches 45 V while keeping the currentconstant. Then, the anodization treatment is ended. In this way, theinsulating film 310 with a thickness of about 50 nm is formed on thesurface of the connection wiring line 305. The numeric values given hereregarding the anodization are merely an example, and the optimal valuesnaturally vary depending on the size of the element to be manufactured,and other factors.

When an anodized film is formed on an aluminum film under theanodization conditions of this embodiment, a Al_(x)O_(y) film with athickness of 51.4 nm is obtained. A 1 mmφ ITO film is formed on theAl_(x)O_(y) film and a voltage of 5 V is applied between the Al film,the Al_(x)O_(y) film, and the ITO film. As a result, a leakage currentas minute as 1×10⁻¹¹ A is measured. This shows that the Al_(x)O_(y) filmcan be used as dielectric of a capacitor storage of a light emittingdevice.

Although the insulating film 310 is formed by anodization here, theinsulating film may be formed by a vapor phase method such as plasmaCVD, thermal CVD, or sputtering. The insulating film may be a siliconoxide film, a silicon nitride film, a silicon nitride oxide film, a DLC(diamond like carbon) film, a tantalum oxide film, or an organicinsulating film. A laminate having the above films in combination may beemployed.

After the insulating film 310 is formed, the conductive film and theinsulating film 310 are patterned into desired shapes to form theconnection wiring line 305, the source line 304, the connection wiringline 320, and the power supply line 321. The source wiring line 304 isconnected, through the contact hole formed in the second interlayerinsulating film 303, the first interlayer insulating film 306, and thegate insulating film 307, to an impurity region 308 of the active layerof the switching TFT 301. Similarly, the connection wiring line 305 isconnected, through the contact hole formed in the second interlayerinsulating film 303, the first interlayer insulating film 306, and thegate insulating film 307, to an impurity region 309 of the active layerof the switching TFT 301.

A third interlayer insulating film 311 is then formed. A part of thethird interlayer insulating film 311 is removed by etching to expose theinsulating film 310 that is formed with being in contact with theconnection wiring line 305. In a separate step, a contact hole reachingthe connection wiring line 320 is formed. At this point, a part of theinsulating film 310 that is in contact with the connection wiring line320 is removed to expose the connection wiring line 320.

Thereafter, a transparent conductive film is formed and etched to form acapacitance wiring line 322 and a pixel electrode 323. The pixelelectrode is connected to the connection wiring line 320 through thecontact hole formed in the third interlayer insulating film 311.

In this embodiment, a storage capacitor 324 is formed from theconnection wiring line 305, the insulating film 310 that is in contactwith the connection wiring line 305, and the capacitance wiring line322.

The storage capacitor structured in accordance with this embodiment haswider choices for the thickness of dielectric and for the dielectricconstant than the one in Embodiment 1.

Embodiment 3

This embodiment describes a case of forming a gate line in the samelayer as a connection wiring line.

FIG. 11 is a sectional view of a pixel of this embodiment. Referencesymbol 301 denotes a switching TFT, and 302, a driving TFT. Denoted by303 and 304 are a source line and a power supply line, respectively.

The source line 303 and the power supply line 304 are formed on a gateinsulating film 307 at the same time a gate electrode 305 of theswitching TFT 301 and a gate electrode 306 of the driving TFT 302 areformed. A capacitance electrode 304 overlaps an impurity region 308 withthe gate insulating film 307 interposed therebetween. The capacitanceelectrode 304, the gate insulating film 307, and the impurity region 308constitute a storage capacitor 309.

Connection wiring lines 311 to 314 and a gate line 330 are formed on asecond interlayer insulating film 310. The connection wiring line 311 isconnected to the source line 303 through a contact hole formed in thesecond interlayer insulating film 310 and a first interlayer insulatingfilm 320. The connection wiring line 314 is connected to the capacitanceelectrode 304 through another contact hole that is formed in the secondinterlayer insulating film 310 and the first interlayer insulating film320.

The connection wiring line 311 is connected to an impurity region 321 ofthe switching TFT 301 through a contact hole formed in the secondinterlayer insulating film 310, the first interlayer insulating film320, and the gate insulating film 307. The connection wiring line 312 isconnected to an impurity region 322 of the switching TFT 301 throughanother contact hole formed in the second interlayer insulating film310, the first interlayer insulating film 320, and the gate insulatingfilm 307. Similarly, through contact holes formed in the secondinterlayer insulating film 310, the first interlayer insulating film320, and the gate insulating film 307, the connection wiring line 313 isconnected to an impurity region 323 of the driving TFT 302 and theconnection wiring line 314 is connected to an impurity region 324 of thedriving TFT 302.

The connection wiring line 312 overlaps the active layer of theswitching TFT with the first and second interlayer insulating films 320and 310 interposed therebetween. Though not shown, the gate line 330 isconnected to the gate electrode 305 of the switching TFT through acontact hole formed in the second interlayer insulating film 310 and thefirst interlayer insulating film 320.

A third interlayer insulating film 340 is formed on the secondinterlayer insulating film 310 so as to cover the connection wiringlines 311 to 314 and the gate line 330. On the third interlayerinsulating film 340, a capacitance wiring line 341 and a pixel electrode342 are formed from the same conductive film. The pixel electrode 342 isconnected to the connection wiring line 313 through a contact holeformed in the third interlayer insulating film 340.

A storage capacitor 343 that is the feature of the present invention isformed from the connection wiring line 312, the third interlayerinsulating film 340, and the capacitance wiring line 341.

By forming the gate line on the same layer as the connection wiring lineas in this embodiment, using different materials to form the gateelectrode and the gate line does not lead to an increase in number ofmanufacture steps. Therefore, it is possible to form the gate electrodefrom a material suitable for fine processing whereas a low resistantmaterial is used to form the gate line.

This embodiment may be combined freely with Embodiment 2.

Embodiment 4

This embodiment describes a pixel structure using a reverse stagger TFT.

FIG. 12 is a sectional view of a pixel of this embodiment. Denoted by401 and 402 are a switching TFT and a driving TFT, respectively.

A source line 405, connection wiring lines 406 and 407, and a powersupply line 408 are formed on a first interlayer insulating film 409.The source line 405 is connected to an impurity region 410 of theswitching TFT 401 through a contact hole formed in the first interlayerinsulating film 409. The connection wiring line 406 is connected to animpurity region 411 of the switching TFT 401 through a contact holeformed in the first interlayer insulating film 409.

The connection wiring line 407 is connected to an impurity region 412 ofthe driving TFT 402 through a contact hole formed in the firstinterlayer insulating film 409. The power supply line 408 is connectedto an impurity region 413 of the driving TFT 402 through a contact holeformed in the first interlayer insulating film 409.

A second interlayer insulating film 415 is formed on the firstinterlayer insulating film 409 so as to cover the source line 405, theconnection wiring lines 406 and 407, and the power supply line 408. Onthe second interlayer insulating film 415, a capacitance wiring line 416and a pixel electrode 417 are formed from the same conductive film. Thepixel electrode 417 is connected to the connection wiring line 407through a contact hole formed in the second interlayer insulating film415.

This embodiment may be combined freely with the structure of Embodiment2.

Embodiment 5

This embodiment describes a case of forming a gate line between anactive layer of a switching TFT and a substrate.

FIG. 13 is a sectional view of a pixel of this embodiment. Denoted by501 and 502 are a switching TFT and a driving TFT, respectively. Alight-shielding film 505 functioning as a gate line is formed between anactive layer 503 of the switching TFT 501 and a substrate 504.

The light-shielding film 505 may be a polysilicon film, a WSi_(x) (x=2.0to 2.8) film, or a film formed of a conductive material such as Al, Ta,W, Cr, and Mo. Alternatively, the light-shielding film may be acombination of these films. In this embodiment, a polysilicon film witha thickness of 50 nm and a WSi_(x) film with a thickness of 100 nm arelayered to form the light-shielding film 505.

A base insulating film 506 is formed between the light-shielding film505 and the active layer 503. The base insulating film 506 is aninsulating film containing silicon (for example, a silicon oxide film, asilicon oxynitride film, and a silicon nitride film) and formed byplasma CVD or sputtering.

In a later step, before forming a gate electrode 507 of the switchingTFT 501, a contact hole reaching the light-shielding film 505 is formedin the base insulating film 506. Then a conductive film for forming thegate electrode 507 is formed. The conductive film is patterned to formthe gate electrode 507, which is connected to the light-shielding film505.

In the above structure, the gate line overlaps the switching TFT 501 andtherefore the aperture ratio is raised.

This embodiment may be combined freely with Embodiment 2.

Embodiment 6

This embodiment describes a method of driving a light emitting device ofthe present invention.

FIG. 14 is a circuit diagram of a pixel portion in a light emittingdevice of the present invention. Reference symbol 601 denotes aswitching TFT, 602, a driving TFT, 603, an OLED, and 604, a storagecapacitor. Details about connection structure in the pixel are the sameas those in the pixel shown in FIG. 1.

The pixel portion has source lines S1 to Sx, power supply lines V1 toVx, and gate lines G1 to Gy. Each pixel has one of the source lines S1to Sx, one of the power supply lines V1 to Vx, and one of the gate linesG1 to Gy.

FIG. 15 is a timing chart of the light emitting device shown in FIG. 14when analog driving is used. A period starting as one gate line isselected and ending as a different gate line is selected next is calledone line period (L). In this specification, selecting a gate line meansthat every TFT whose gate electrode is connected to the selected gateline is turned ON.

A period starting as one image is displayed and ending as the next imageis displayed corresponds to one frame period (F). The light emittingdevice shown in FIG. 14 has y gate lines. Therefore one frame period hasy line periods (L1 to Ly).

The electric potential of the power supply lines (V1 to Vx) (powersupply electric potential) is kept constant. The electric potential ofan opposite electrode is also kept constant. The electric potentialdifference between the opposite electrode and the power supply lines islarge enough to cause an OLED to emit light when the power supplyelectric potential is given to a pixel electrode of the OLED.

In the first line period (L1), the gate line G1 is selected in responseto a selection signal to turn every switching TFT 601 that is connectedto the gate line G1 ON. Then analog video signals are inputted to thesource lines (S1 to Sx) in order. The analog video signals inputted tothe source lines (S1 to Sx) are then inputted to a gate electrode of thedriving TFT 602 through the switching TFT 601.

The amount of current flowing in a channel forming region of the drivingTFT 602 is controlled by a gate voltage VGS, which represents theelectric potential difference between the gate electrode of the drivingTFT 602 and a source region thereof. Therefore the electric potentialgiven to the pixel electrode of the OLED 603 is determined by the levelof electric potential of an analog video signal inputted to the gateelectrode of the driving TFT 602. The OLED 603 thus emits light at aluminance controlled by the electric potential of the analog videosignal.

The operation described above is repeated until inputting analog videosignals to all of the source lines (S1 to Sx) is finished. Then, thefirst line period (L1) is completed. One line period may alternativelybe defined as a period required to finish inputting analog video signalsto the source lines (S1 to Sx) plus a horizontal retrace period. Next,the second line period (L2) is started and the gate line G2 is selectedin response to a selection signal. Then, similar to the first lineperiod (L1), analog video signals are inputted to the source lines (S1to Sx) in order.

After every gate line (every one of G1 to Gy) is selected once, all ofthe line periods (L1 to Ly) are completed. Completion of all the lineperiods (L1 to Ly) brings completion of one frame period. In one frameperiod, all pixels are used to display and one image is formed. Oneframe period may alternatively be defined as all line periods (L1 to Ly)plus vertical retrace periods.

As described above, the luminance of an OLED is controlled by theelectric potential of an analog video signal in analog driving, and grayscale display is obtained through the control of the luminance.

In analog driving, the capacity of a storage capacitor is desirablylarger than in digital driving. Therefore, the structure of the lightemitting device of the present invention, in which the storage capacitorcan have a large capacity while avoiding lowering of the aperture ratio,is suitable for analog driving. However, the present invention is notlimited to this driving method and the present invention can fully beapplied to a digitally-driven light emitting device.

This embodiment may be combined freely with Embodiments 1 through 5.

Embodiment 7

This embodiment describes a method of driving a light emitting devicestructured as shown in FIG. 14 which is different from the drivingmethod of Embodiment 6.

A light emitting device of this embodiment displays an image using adigital video signal that carries image information. FIG. 16 is a timingchart showing points at which writing periods and light emission periodsare started in digital driving. In FIG. 16, the axis of abscissaindicates time whereas the axis of ordinate indicates positions ofpixels of the respective lines.

First, the power supply electric potential of the power supply lines (V1to Vx) is kept at the same level as the electric potential of theopposite electrode of the OLED 603. The gate line G1 is selected inresponse to a selection signal to turn the switching TFT 601 of everypixel that is connected to the gate line G1 (every pixel on Line One)ON.

A first set of one bit digital video signals are inputted to the sourcelines (S1 to Sx). The digital video signals are then inputted to thegate electrode of the driving TFT 602 through the switching TFT 601.

When the gate line G1 is no longer selected, the gate line G2 isselected to turn the switching TFT 601 in every pixel that is connectedto the gate line G2 ON. Then, the first set of one bit digital videosignals are inputted from the source lines (S1 to Sx) to the pixels onLine Two.

In this way, all the gate lines (G1 to Gy) are selected one by one. Aperiod it takes for the first set of one bit digital video signals to beinputted to the pixels of all the lines after all the gate lines (G1 toGy) are selected is a writing period Ta1.

As the writing period Ta1 is ended, a display period Tr1 is started. Inthe display period Tr1, the power supply electric potential of the powersupply lines is set to a level that produces an electric potentialdifference with the electric potential of the opposite electrode largeenough to cause the OLED to emit light when the power supply electricpotential is given to the pixel electrode of the OLED.

In the display period Tr1, whether the OLED 603 emits light or not isdetermined by the digital video signals that have been written in thepixels in the writing period Ta1. The driving TFT 602 is turned OFF whena digital video signal contains information of ‘0’. Accordingly, thepower supply electric potential is not given to the pixel electrode ofthe OLED 603. Therefore no light is emitted from the OLED 603 of a pixelto which a digital video signal having information of ‘0’ is inputted.When a digital video signal has information of ‘1’, on the other hand,the driving TFT 602 is turned ON. Then, the power supply electricpotential is given to the pixel electrode of the OLED 603. Therefore,light is emitted from the OLED 603 of a pixel to which a digital videosignal having information of ‘1’ is inputted.

Thus, the OLED 603 emits or does not emit light in the display periodTr1 and all the pixels are used to display.

As the display period Tr1 is ended, a writing period Ta2 is started andthe power supply electric potential of the power supply lines is set tothe same level as the electric potential of the opposite electrode ofthe OLED. Similar to the writing period Ta1, all the gate lines areselected one by one and a second set of one bit digital video signalsare inputted to all of the pixels in order. The writing period Ta2 is aperiod it takes for the second set of one bit digital video signals tobe inputted to pixels of all the lines.

As the writing period Ta2 is ended, a display period Tr2 is started. Inthis period, the power supply electric potential of the power supplylines is set to a level that produces an electric potential differencewith the electric potential of the opposite electrode large enough tocause the OLED 603 to emit light when the power supply electricpotential is given to the pixel electrode of the OLED 603. Then, all ofthe pixels are used to display.

The above operation is repeated until inputting an n-th set of one bitdigital video signals to the pixels is finished, alternating one writingperiod Ta with one display period Tr. When all display periods (Tr1 toTrn) are completed, one image is displayed. In the driving method ofthis embodiment, a period required to display one image is called oneframe period (F). When one frame period is ended, the next frame periodis started. Then, the writing period Ta1 is started again to repeat theoperation described above.

In a usual light emitting device, preferably sixty or more frame periodsare provided in one second. If the number of images displayed in onesecond is less than sixty, flickering of image may become noticeable tothe eye.

In this embodiment, the lengths of all writing periods in total have tobe shorter than the length of one frame period and it is necessary toset the ratio of lengths of display periods to satisfy Tr1:Tr2:Tr3: . .. :Tr(n−1): Trn=2⁰:2¹:2₂(n−2):2^((n−)1). A desired gray scale out of2^(n) gray scales can be displayed by combining the display periods.

The gray scale of a pixel in one frame period is determined by the sumof lengths of display periods in the one frame period in which the OLEDof that pixel emits light. For example, if n=8, a pixel obtains 100% ofluminance when the pixel emits light in all display periods. When thepixel emits light in Tr1 and Tr2, the luminance thereof is 1%. When thepixel emits light in Tr3, Tr5, and Tr8, the luminance thereof is 60%.

The display periods Tr1 to Trn may be run in any order. For instance, inone frame period started by Tr1, Tr3, Tr5, Tr2, . . . may follow Tr1 inthis order.

In this embodiment, the power supply electric potential of the powersupply lines is set to different levels in a writing period and in adisplay period. However, the present invention is not limited thereto.The power supply electric potential and the electric potential of theopposite electrode may always keep an electric potential differencelarge enough to cause the OLED to emit light when the power supplyelectric potential is given to the pixel electrode of the OLED. In thiscase, the OLED can emit light also in a writing period. Then, the grayscale of a pixel in one frame period is determined by the sum of lengthsof writing periods and display periods in the one frame period in whichthe OLED of that pixel emits light. Here, lengths of writing periods anddisplay periods for digital video signals of the respective bits have tobe set to satisfy the following ratio. (Ta1+Tr1):(Ta2+Tr2):(Ta3+Tr3): .. . :(Ta(n−1)+Tr(n−1)):(Tan+Trn)=2⁰:2¹:2²: . . . :2^((n−2)):2^((n−1)).

This embodiment may be combined freely with Embodiments 1 through 5.

Embodiment 8

This embodiment describes with reference to FIGS. 17A to 17C a case ofmanufacturing a light emitting device using the present invention.

FIG. 17A is a top view of a light emitting device in which a substratewith an OLED formed thereon is sealed by a sealing member. FIG. 17B is asectional view taken along the line A-A′ in FIG. 17A. FIG. 17C is asectional view taken along the line B-B′ in FIG. 17A.

A pixel portion 4002, a source line driving circuit 4003, and first andsecond gate line driving circuits 4004 a and 4004 b are formed on asubstrate 4001. A seal member 4009 is placed so as to surround them allon the substrate. A sealing member 4008 is provided on the pixel portion4002, the source line driving circuit 4003, and the first and secondgate line driving circuits 4004 a and 4004 b. Accordingly, the pixelportion 4002, the source line driving circuit 4003, and the first andsecond gate line driving circuits 4004 a and 4004 b are sealed in thespace defined by the substrate 4001, the seal member 4009, and thesealing member 4008, with a filler 4210 filling the space.

The pixel portion 4002, the source line driving circuit 4003, and thefirst and second gate line driving circuits 4004 a and 4004 b on thesubstrate 4001 each have a plurality of TFTs. The source line drivingcircuit 4003 is a circuit for inputting video signals to source lines.The first and second gate line driving circuits 4004 a and 4004 b arecircuits for selecting a gate line in response to a selection signal.

FIG. 17B shows, as representatives of those TFTs, a driving circuit TFT(composed of an n-channel TFT and a p-channel TFT in FIG. 17B) 4201included in the source line driving circuit 4003 and a driving TFT (aTFT for controlling a current flowing into the OLED) 4202 included inthe pixel portion 4002. The TFTs 4201 and 4202 are formed on a base film4010.

In this embodiment, the n-channel TFT or the p-channel TFT thatconstitutes the driving circuit TFT 4201 is manufactured by a knownmethod, and a p-channel TFT manufactured by a known method is used forthe driving TFT 4202. The pixel portion 4002 is provided with a storagecapacitor (not shown) connected to a gate of the driving TFT 4202.

Formed on the driving circuit TFT 4201 and the driving TFT 4202 is aninterlayer insulating film (planarization film) 4301, on which a pixelelectrode (anode) 4203 is formed to be electrically connected to a drainof the driving TFT 4202. The pixel electrode 4203 is formed of atransparent conductive film having a large work function. Examples ofthe usable transparent conductive film material include a compound ofindium oxide and tin oxide, a compound of indium oxide and zinc oxide,zinc oxide alone, tin oxide alone, and indium oxide alone. A transparentconductive film formed of one of these materials and doped with galliummay also be used for the pixel electrode.

An insulating film 4302 is formed on the pixel electrode 4203. Anopening is formed in the insulating film 4302 above the pixel electrode4203. At the opening above the pixel electrode 4203, an organic lightemitting layer 4204 is formed. The organic light emitting layer 4204 isformed of a known organic luminous material or inorganic luminousmaterial. Either low molecular weight (monomer) organic luminousmaterials or high molecular weight (polymer) organic luminous materialscan be used for the organic light emitting layer.

The organic light emitting layer 4204 is formed by a known evaporationtechnique or application technique. The organic light emitting layer mayconsist solely of a light emitting layer. Alternatively, the organiclight emitting layer may be a laminate having, in addition to a lightemitting layer, a hole injection layer, a hole transporting layer, anelectron transporting layer, and an electron injection layer in anycombination.

A cathode 4205 is formed on the organic light emitting layer 4204 from alight-shielding conductive film (typically, a conductive film mainlycontaining aluminum, copper, or silver, or a laminate consisting of theabove conductive film and other conductive films). Desirably, moistureand oxygen are removed as much as possible from the interface betweenthe cathode 4205 and the organic light emitting layer 4204. Somecontrivance is needed for the removal. For example, the organic lightemitting layer 4204 is formed in a nitrogen or rare gas atmosphere andthen the cathode 4205 is successively formed without exposing thesubstrate to moisture and oxygen. This embodiment uses a multi-chambersystem (cluster tool system) film formation apparatus to achieve thefilm formation described above. The cathode 4205 receives a givenvoltage.

An OLED 4303 composed of the pixel electrode (anode) 4203, the organiclight emitting layer 4204, and the cathode 4205 is thus formed. Aprotective film 4209 is formed on the insulating film 4302 so as tocover the OLED 4303. The protective film 4209 is effective in preventingoxygen and moisture from entering the OLED 4303.

Denoted by 4005 a is a lead-out wiring line connected to a power supplyline, and is electrically connected to a source region of the drivingTFT 4202. The lead-out wiring line 4005 a runs between the seal member4009 and the substrate 4001 and is electrically connected to an FPCwiring line 4301 of an FPC 4006 through an anisotropic conductive film4300.

The sealing member 4008 is formed of a glass material, a metal material(typically a stainless steel material), a ceramic material, or a plasticmaterial (including a plastic film). Examples of the usable plasticmaterial include an FRP (fiberglass-reinforced plastic) plate, a PVF(polyvinyl fluoride) film, a Mylar film, a polyester film, and anacrylic resin film. A sheet obtained by sandwiching an aluminum foilbetween PVF films or Mylar films may also be used.

However, if light emitted from the OLED travels toward the sealingmember, the sealing member has to be transparent. In this case, atransparent material such as a glass plate, a plastic plate, a polyesterfilm, or an acrylic film is used.

The filler 4210 may be inert gas such as nitrogen and argon, or aUV-curable resin or a thermally curable resin. Examples thereof includePVC (polyvinyl chloride), acrylic, polyimide, an epoxy resin, a siliconeresin, PVB (polyvinyl butylal), and EVA (ethylene vinyl acetate). Inthis embodiment, nitrogen is used as the filler.

In order to expose the filler 4210 to a hygroscopic substance(preferably barium oxide) or a substance capable of adsorbing oxygen, ahygroscopic substance 4207, or a substance 4207 capable of adsorbingoxygen, is placed in a convex portion 4007 formed on a surface of thesealing member 4008 on the substrate 4001 side. The hygroscopicsubstance 4207, or a substance 4207 capable of adsorbing oxygen, is helddown to the convex portion 4007 by a concave portion covering member4208 to prevent hygroscopic substance 4207, or a substance 4207 capableof adsorbing oxygen, from scattering. The convex portion covering member4208 is a dense mesh and allows air and moisture to pass but not thehygroscopic substance 4207, or a substance 4207 capable of adsorbingoxygen. The hygroscopic substance 4207, or a substance 4207 capable ofadsorbing oxygen, can prevent degradation of the OLED 4303.

As shown in FIG. 17C, a conductive film 4203 a is formed to be broughtinto contact with the top face of the lead-out wiring line 4005 a at thesame time the pixel electrode 4203 is formed.

The anisotropic conductive film 4300 has a conductive filler 4300 a. Theconductive filler 4300 a electrically connects the conductive film 4203a on the substrate 4001 to the FPC wiring line 4301 on the FPC 4006 uponthermal press fitting of the substrate 4001 and the FPC 4006.

This embodiment may be combined freely with Embodiments 1 through 7.

Embodiment 9

In this embodiment, an external light emitting quantum efficiency can beremarkably improved by using an organic light emitting material by whichphosphorescence from a triplet exciton can be employed for emitting alight. As a result, the power consumption of the OLED can be reduced,the lifetime of the OLED can be elongated and the weight of the OLED canbe lightened.

The following is a report where the external light emitting quantumefficiency is improved by using the triplet exciton (T. Tsutsui, C.Adachi. S. Saito, Photochemical processes in Organized MolecularSystems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437).

The molecular formula of an organic light emitting material (coumarinpigment) reported by the above article is represented as follows.

(M. A. Baldo, D. F. O Brien, Y. You, A. Shoustikov, S. Sibley. M. E.Thompson. S. R. Forrest, Nature 395 (1998) p. 151)

The molecular formula of an organic light emitting material (Pt complex)reported by the above article is represented as follows.

(M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest,Appl. Phys. Lett., 75 (1999) p. 4.)(T. Tsutsui, M.-J. Yang, M. Yahiro. K. Nakamura, T. Watanabe. T. TsujiY. Fukuda. T. Wakimoto, S. Mayaguchi, Jpn, Appl. Phys., 38 (12B) (1999)L1502)

The molecular formula of an organic light emitting material (Ir complex)reported by the above article is represented as follows.

As described above, if phosphorescence from a triplet exciton can be putto practical use, it can realize the external light emitting quantumefficiency three to four times as high as that in the case of usingfluorescence from a singlet exciton in principle.

The structure according to this embodiment can be freely implemented incombination of any structures of Embodiments 1 to 8.

Embodiment 10

This embodiment describes a storage capacitor of the present inventionwhich has a structure different from the one shown in FIG. 2.

FIG. 21 is a sectional view of a pixel of this embodiment. Componentscommon to those in FIG. 2 are denoted by the same symbols.

On the second interlayer insulating film 117, the source line (S), theconnection wiring lines 118 and 119, and the power supply line (V) areformed. The source line (S) is connected to the impurity region 110through a contact hole that is formed in the second interlayerinsulating film 117. The connection wiring line 118 is connected to theimpurity region 111 through a contact hole that is formed in the secondinterlayer insulating film 117. The connection wiring line 119 isconnected to the impurity region 112 through a contact hole that isformed in the second interlayer insulating film 117. The power supplyline (V) is connected to the impurity region 113 through a contact holethat is formed in the second interlayer insulating film 117. Theconnection wiring line 118 overlaps the active layer 130 with the secondinterlayer insulating film 117 interposed therebetween.

A capacitance insulating film 170 is formed on the second interlayerinsulating film 117 so as to cover the source line (S), the connectionwiring lines 118 and 119, and the power supply line (V). The material ofthe capacitance insulating film 170 may be either inorganic or organicas long as the material is capable of insulating. However, the materialhas to have a different selective ratio in etching from the selectiveratio of the subsequently formed third interlayer insulating film 120.

The third interlayer insulating film 120 is formed on the capacitanceinsulating film 170. A part of the third interlayer insulating filmwhere it overlaps the connection wiring line 118 is removed throughetching to expose the capacitance insulating film 170. When acapacitance wiring line 121 is formed later, this structure brines theconnection wiring line 118, the capacitance insulating film 170, and thecapacitance wiring line 121, which are formed in this order, intocontact with one another.

On the third interlayer insulating film 120, the capacitance wiring line121 and a pixel electrode 122 are formed.

The pixel electrode 122 is connected to the connection wiring line 119through a contact hole that is formed in the third interlayer insulatingfilm 120.

In this embodiment, the storage capacitor 104 is formed in an area wherethe capacitance insulating film 170 is sandwiched between the connectionwiring line 118 and the capacitance wiring line 121. The capacitanceinsulating film 170 is described in this embodiment as a layer separatefrom the third interlayer insulating film 120. However, the capacitanceinsulating film 170 may be regarded as a part of the third interlayerinsulating film 120 that is comprised of layers of insulating films.

The structure of this embodiment may be combined with any of thestructures of Embodiment 1 and Embodiments 3 through 9.

Embodiment 11

Being self-luminous, a light emitting device has better visibility inbright places and wider viewing angle than liquid crystal displaydevices. Therefore the light emitting device can be used for displayunits of various electric appliances.

Given as examples of an electric appliance that employs a light emittingdevice manufactured in accordance with the present invention are videocameras, digital cameras, goggle type displays (head mounted displays),navigation systems, audio reproducing devices (such as car audio andaudio components), notebook computers, game machines, portableinformation terminals (such as mobile computers, cellular phones,portable game machines, and electronic books), and image reproducingdevices equipped with recording media (specifically, devices with adisplay device that can reproduce data in a recording medium such as adigital versatile disk (DVD) to display an image of the data). Wideviewing angle is important particularly for portable informationterminals because their screens are often slanted when they are lookedat. Therefore it is preferable for portable information terminals toemploy the light emitting device using the organic light emittingelement. Specific examples of these electric appliance are shown inFIGS. 18A to 18H.

FIG. 18A shows an OLED display device, which is composed of a case 2001,a support base 2002, a display unit 2003, speaker units 2004, a videoinput terminal 2005, etc. the light emitting device manufactured inaccordance with the present invention can be applied to the display unit2003. Since the light emitting device is self-luminous, the device doesnot need back light and can make a thinner display unit than liquidcrystal display devices. The OLED display device refers to all displaydevices for displaying information, including ones for personalcomputers, for TV broadcasting reception, and for advertisement.

FIG. 18B shows a digital still camera, which is composed of a main body2101, a display unit 2102, an image receiving unit 2103, operation keys2104, an external connection port 2105, a shutter 2106, etc. The lightemitting device manufactured in accordance with the present inventioncan be applied to the display unit 2102.

FIG. 18C shows a notebook personal computer, which is composed of a mainbody 2201, a case 2202, a display unit 2203, a keyboard 2204, anexternal connection port 2205, a pointing mouse 2206, etc. The lightemitting device manufactured in accordance with the present inventioncan be applied to the display unit 2203.

FIG. 18D shows a mobile computer, which is composed of a main body 2301,a display unit 2302, a switch 2303, operation keys 2304, an infraredport 2103. etc. The light emitting device manufactured in accordancewith the present invention can be applied to the display unit 2302.

FIG. 18E shows a portable image reproducing device equipped with arecording medium (a DVD player, to be specific). The device is composedof a main body 2401, a case 2402, a display unit A 2403, a display unitB 2404, a recording medium (DVD or the like) reading unit 2405,operation keys 2406, speaker units 2407. etc. The display unit A 2403mainly displays image information whereas the display unit B 2404 mainlydisplays text information. The light emitting device manufactured inaccordance with the present invention can be applied to the displayunits A 2403 and B 2404. The image reproducing device equipped with arecording medium also includes home-video game machines.

FIG. 18F shows a goggle type display (head mounted display), which iscomposed of a main body 2501, display units 2502, and arm units 2503.The light emitting device manufactured in accordance with the presentinvention can be applied to the display units 2502.

FIG. 18G shows a video camera, which is composed of a main body 2601, adisplay unit 2602, a case 2603, an external connection port 2604, aremote control receiving unit 2605, an image receiving unit 2606, abattery 2607, an audio input unit 2608, operation keys 2609. etc. Thelight emitting device manufactured in accordance with the presentinvention can be applied to the display unit 2602.

FIG. 18H shows a cellular phone, which is composed of a main body 2701,a case 2702, a display unit 2703, an audio input unit 2704, an audiooutput unit 2705, operation keys 2706, an external connection port 2707,an antenna 2708, etc. The light emitting device manufactured inaccordance with the present invention can be applied to the display unit2703. If the display unit 2703 displays white letters on blackbackground, the cellular phone consumes less power.

If the luminance of light emitted from organic materials is raised infuture, the light emitting device can be used in front or rearprojectors by enlarging outputted light that contains image informationthrough a lens or the like and projecting the light.

These electric appliances now display with increasing frequencyinformation sent through electronic communication lines such as theInternet and CATV (cable television), especially, animation information.Since organic light emitting materials have very fast response speed,the light emitting device is suitable for animation display.

In the light emitting device, light emitting portions consume power andtherefore it is preferable to display information in a manner thatrequires less light emitting portions. When using the light emittingdevice in display units of portable information terminals, particularlycellular phones and audio reproducing devices that mainly display textinformation, it is preferable to drive the device such that non-lightemitting portions form a background and light emitting portions formtext information.

As described above, the application range of the light emitting devicemanufactured in accordance with the present invention is so wide that itis applicable to electric appliances of any field. The electricappliances of this embodiment can employ any light emitting devicedisclosed in Embodiments 1 through 9.

With the above structure, the present invention allows the storagecapacitor to overlap the TFT, thereby increasing the capacity of thestorage capacitor while keeping the aperture ratio from lowering.Accordingly, a change in gate voltage due to leakage or other causes canbe avoided to prevent a change in luminance of an OLED and flickering ofscreen in analog driving.

Keeping the aperture ratio from lowering also leads to preventing thearea of effective light emission of a pixel from being reduced. As thearea of effective light emission is larger, the luminance of the screenis higher. Therefore, the structure of the present invention iseffective in reducing power consumption.

1. A light emitting device comprising: a base film formed over asubstrate; a switching thin film transistor comprising a firstsemiconductor film and a gate electrode over the first semiconductorfilm with a gate insulating film interposed therebetween, the firstsemiconductor film being in contact with the base film and comprising asource region and a drain region; a driving thin film transistor overthe substrate; a power supply line comprising a conductive film incontact with the gate insulating film; a first capacitor comprising asecond semiconductor film and the conductive film over the secondsemiconductor film, the second semiconductor film being in contact withthe base film, wherein the conductive film is connected to one of asource region and a drain region of the driving thin film transistorthrough a connection wiring; an insulating film over the switching thinfilm transistor and the first capacitor; a second capacitor comprising awiring and a transparent conductive film, the wiring being electricallyconnected to one of the source region and the drain region of theswitching thin film transistor, and the transparent conductive filmbeing in contact with the insulating film; and a light emitting elementcomprising a pixel electrode and a light emitting layer formed over thepixel electrode, the pixel electrode being in contact with theinsulating film.
 2. The light emitting device according to claim 1,wherein the gate electrode and the conductive film comprise the samematerial.
 3. The light emitting device according to claim 1, wherein thetransparent conductive film and the pixel electrode comprise the samematerial.
 4. The light emitting device according to claim 1, wherein thetransparent conductive film is separate and distinct from the pixelelectrode.
 5. The light emitting device according to claim 1, whereinthe wiring and the transparent conductive film overlap each other. 6.The light emitting device according to claim 1, wherein the lightemitting device is incorporated in one selected from the groupconsisting of a display device, a camera, a computer, an imagereproducing device, a goggle type display, and a cellular phone.
 7. Alight emitting device comprising: a switching thin film transistor overa substrate, the switching thin film transistor comprising a firstsemiconductor film, a first gate electrode and a second gate electrode,the first and second gate electrodes being formed over the firstsemiconductor film with a gate insulating film interposed therebetween,and the first semiconductor film comprising a source region and a drainregion; a driving thin film transistor over the substrate; a powersupply line comprising a conductive film in contact with the gateinsulating film; a first capacitor over the substrate, the firstcapacitor comprising a second semiconductor film and the conductive filmover the second semiconductor film, wherein the conductive film isconnected to one of a source region and a drain region of the drivingthin film transistor through a connection wiring; an insulating filmover the switching thin film transistor and the first capacitor; asecond capacitor comprising a wiring and a transparent conductive film,the wiring being electrically connected to one of the source region andthe drain region of the switching thin film transistor, and thetransparent conductive film being in contact with the insulating film;and a light emitting element comprising a pixel electrode and a lightemitting layer formed over the pixel electrode, the pixel electrodebeing in contact with the insulating film.
 8. The light emitting deviceaccording to claim 7, wherein the first gate electrode, the second gateelectrode and the conductive film comprise the same material.
 9. Thelight emitting device according to claim 7, wherein the transparentconductive film and the pixel electrode comprise the same material. 10.The light emitting device according to claim 7, wherein the transparentconductive film is separate and distinct from the pixel electrode. 11.The light emitting device according to claim 7, wherein the wiring andthe transparent conductive film overlap each other.
 12. The lightemitting device according to claim 7, wherein the light emitting deviceis incorporated in one selected from the group consisting of a displaydevice, a camera, a computer, an image reproducing device, a goggle typedisplay, and a cellular phone.
 13. A light emitting device comprising: aswitching thin film transistor over a substrate, the switching thin filmtransistor comprising a first semiconductor film and a gate electrodeover the first semiconductor film with a gate insulating film interposedtherebetween, the first semiconductor film comprising a source regionand a drain region; a driving thin film transistor over the substrate; apower supply line comprising a conductive film in contact with the gateinsulating film; a first capacitor over the substrate, the firstcapacitor comprising a second semiconductor film and the conductive filmover the second semiconductor film, wherein the conductive film isconnected to one of a source region and a drain region of the drivingthin film transistor through a connection wiring; an insulating filmover the switching thin film transistor and the first capacitor; asecond capacitor comprising a wiring and a transparent conductive film,the wiring being electrically connected to one of the source region andthe drain region of the switching thin film transistor, and thetransparent conductive film being in contact with the insulating film;and a light emitting element comprising a pixel electrode and a lightemitting layer formed over the pixel electrode, the pixel electrodebeing in contact with the insulating film and the light emitting elementbeing electrically connected to the driving thin film transistor. 14.The light emitting device according to claim 13, wherein the gateelectrode and the conductive film comprise the same material.
 15. Thelight emitting device according to claim 13, wherein the transparentconductive film and the pixel electrode comprise the same material. 16.The light emitting device according to claim 13, wherein the transparentconductive film is separate and distinct from the pixel electrode. 17.The light emitting device according to claim 13, wherein the wiring andthe transparent conductive film overlap each other.
 18. The lightemitting device according to claim 13, wherein the light emitting deviceis incorporated in one selected from the group consisting of a displaydevice, a camera, a computer, an image reproducing device, a goggle typedisplay, and a cellular phone.